Modern integrated circuits have become increasingly complex, to a point where they now contain the equivalent of several hundred thousand transistors. The increased heat generated by these advanced integrated circuits must be efficiently removed in order to keep them from suffering a high failure rate caused by high temperature operation.
Particularly troublesome is the problem of transferring heat from semiconductor devices and other heat generating devices which are mounted to a substrate such as a printed circuit board. Since printed circuit boards are by their very nature poor conductors of heat, the devices must be cooled using additional means such as a fan or a heat sink placed on or near the heat generating device. It is difficult, however, to place a heat sink on or near a device which itself establishes a highly irregular surface. Additionally, cooling requirements are rendered more difficult by improvements in component packaging densities, as well as increased use of integrated circuits in environments where extreme temperatures and mechanical packaging difficulties exist, such as in aerospace electronics. Improvements to resolve these thermal and mechanical difficulties would allow larger printed circuit board modules, reduce system size and temperatures, and thereby improve system reliability.
In many applications, designers of printed circuit boards simply rely on the copper traces, ground plane, and power plane located in the printed circuit board to transfer the heat. The addition of copper planes or metal cores to the printed circuit board is also used. However, reliance on the copper in the printed circuit board is only satisfactory when heat generation of the electronic devices and ambient environmental temperatures are low, so that these poorer thermal paths can be tolerated. Moreover, the addition of copper planes or metal cores to the printed circuit board increases the cost and complexity of the board.
Another method of transferring the heat from the components is to laminate a heat sink to the printed circuit board. Laminating a heat sink to the component side of a printed circuit board requires cutouts in the heat sink to accommodate the component leads. The heat sink with cutouts is then laminated to the component side surface of the printed circuit board. The component leads are then inserted into the circuit board and the body may be cemented to the heat sink. Much heat sink efficiency is lost due to the cutouts and, in addition, component leads may not be long enough to protrude through the solder side of the circuit board because the components are raised above the board by the thickness of the heat sink. Additionally, the increased use of components utilizing the leaded array package, whose leads may occupy nearly all the surface area under component, makes it impossible to provide a continuous heat sink under the component. These packages need adequate heat sinking the most, since they are, by nature, large and tend to be heat generating devices.
Lamination of a heat sink to the back of a printed circuit board has previously been limited to printed circuit boards with surface mounted components whose leads by definition do not protrude through the backside of the board or if attempted with through-the-board components, would be characterized as nearly ineffective. Thermal contact of the heat sink with the back of the printed circuit board is limited, at best, due to the irregular surface formed by the component leads protruding through the back of the board.